Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia

Therizinosaurs were a group of herbivorous/omnivorous theropod dinosaurs that lived in Asia and North America during the Cretaceous Period. These theropods are notable for their three-fingered hands sporting large claw-like unguals. Here, we describe a new and unusual species of the therizinosaurid Duonychus tsogtbaatari, recovered from the lower Upper Cretaceous Bayanshiree Formation of the Gobi Desert of Mongolia. This species is different from other therizinosaurs in that the hand possesses only two fingers, rather than three fingers, the typical condition for Avetheropoda, providing further insight into forelimb/hand reduction among theropods. Phylogenetically, Duonychus reveals a broader but still limited appearance of didactyly among avetheropodans. The manual ungual of Duonychus tsogtbaatari also preserves a nearly complete three-dimensional structure covering the ungual of the left manual digit I, which represents a keratinous manual claw. The strong curvature of this large claw and high ungual flexion indicate that Duonychus was likely amplectorial (grasping) and an effective grasper of vegetation, despite having only two functional fingers.镰刀龙类（Therizinosaurs）是一类生存于白垩纪（Cretaceous Period）的植食性/杂食性兽脚类恐龙，其化石分布于亚洲与北美地区。该类兽脚类最显著的特征为手部具三根手指，指端带有大型爪状爪骨（ungual）。本研究描述了一类全新且形态特殊的镰刀龙科（Therizinosauridae）物种——杜氏爪龙（Duonychus tsogtbaatari），其化石发现于蒙古国戈壁沙漠的上白垩统下部巴彦希勒组（Bayanshiree Formation）地层中。该物种与其他镰刀龙类的区别在于其手部仅具两根功能性手指，而非真兽脚类（Avetheropoda）典型的三根手指，这一发现为揭示兽脚类恐龙前肢/手部的退化演化过程提供了新的研究视角。系统发育分析表明，杜氏爪龙揭示出真兽脚类类群中两指性的出现范围更广，但仍相对局限。杜氏爪龙的手部爪骨还保存了近乎完整的三维结构，覆盖了左手第一指的爪骨，该结构即为角质爪鞘。该大型爪骨强烈的弯曲度与较高的爪骨屈伸角度表明，尽管仅具有两根功能性手指，杜氏爪龙大概率具备抱握（grasping）能力，且能够高效抓取植物性食物。